JPS6043429B2 - Alloyed galvanized steel sheet with excellent weldability - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an alloyed galvanized steel sheet with excellent weldability.

As is well known, galvanized steel sheets have difficulties in welding.
Therefore, galvanized steel sheets that have been alloyed or have a small amount of coating are used, but even with these galvanized steel sheets that have been alloyed or have a small amount of coating, welding workability is inferior to the welding process. There are cases.

That is, at the initial stage of welding, the plating layer melts and spreads, reducing the current density, and as a result, it is necessary to increase the welding current or lengthen the welding time.

Also, the welding electrode and the plating layer become alloyed, resulting in a decrease in high-temperature hardness and C.
The resistance of the u-Zn alloy increases, and it does not contribute to so-called nugget formation, and the Cu-Zn alloy is deposited on the electrode, causing a decrease in current density as the electrode diameter increases, and as the number of welding points increases, nuggets are formed. It becomes difficult.

The present invention has been made to advantageously solve these difficulties, and its special feature is that it is possible to form crease welds on alloyed galvanized steel sheets with a surface roughness of 1.3 to 3.5 μmrms. This invention relates to alloyed galvanized steel sheets with excellent properties.

As mentioned above, the method of forming the surface roughness of 1.3 to 3.5 μm rms is, for example, by roll rolling, brushing, plasting, etc. on the plated original plate (steel strip).
A roughness of 3.5 μmrms is imparted, and then plated to a coating weight of 30 to 120 g/I by hot-dip galvanizing.
This is heated and alloyed (zinc-iron diffusion reaction), and then subjected to temper rolling to give a surface roughness of 1.3 to 3.5 μmrms.

That is, the roughness imparted to the plating original plate is 1.3 μmrm.
If it is less than s, it is difficult to make the surface roughness of the alloyed plated steel sheet 1.3 to 3.5 μm rms,
Further, if the rolling speed is 3.5 μmrms or more, when forming the roughness by rolling, there will be problems such as severe wear of the rolls and poor productivity.

Next, if the surface roughness of the alloyed plated steel sheet is less than 1.3, there will be little improvement in weldability, and if it is more than 3.5μ1mls, the paint will be sucked in during the final painting process, resulting in poor paint finish. Undesirable. That is, as shown in the figure, about 1.
It has been shown that the weldability of alloyed galvanized steel sheets imparted with a surface J roughness of 3 μm rms or more is significantly improved. The reason for this is thought to be that the Zn oxide layer on the surface of the alloy layer of the steel sheet whose roughness has been adjusted to the above range is extremely small, and alloying between the electrode and the plating layer during spot welding is suppressed. .

In addition, normal welding is carried out within a current range in which nuggets are formed and welding between the electrode and the steel plate does not occur, but of course this current range varies depending on the thickness of the plated original plate, and the current range varies depending on the plate thickness. The optimal value of is adopted.

However, the reactivity between the electrode and the plating layer is the dominant factor in the continuous dot performance, and the plate thickness has little effect on the continuous dot performance in the commonly used plate thickness range of 0.6 to 2.3 mm. Additionally, as the plating thickness increases, the amount of heat of fusion increases and a portion of the heat input is consumed, which increases the lower limit of the current required for nugget formation, but normal welding can be performed by setting the current value accordingly. However, the generally used plating amount range of 30 to 120 g/d does not have a large effect on continuous dot performance.

Continuous dotting is a phenomenon in which the reaction at the interface between the electrode and the plating layer is the dominant factor.

We believe that the reason why surface roughness has a large effect on alloyed galvanized steel sheets is as follows. That is, the alloyed hot-dip galvanized steel sheet consists of a Fe--Zn alloy phase up to the surface of the plating layer, and has a relatively high melting point.

Therefore, melting starts at a relatively high temperature, and the gas trapped in the rough surface directly under the electrode is likely to explode, causing a so-called dust phenomenon. It is surmised that the contaminated layer or Cu-Zn alloy layer on the electrode surface is self-dressed when this "dust" is generated, so that the electrode surface remains clean during continuous dots.This self-dressing Since dust tends to occur at the circumferential portion of the electrode tip, the diameter of the electrode tip does not increase due to dust, which is convenient.

If the surface roughness is less than 1.3 μm-Rms, the self-dressing effect will not be sufficient, and if continuous welding is continued, the Cu
-Zn alloy grows and eventually becomes unweldable.

The roughness of the plated original sheet is determined by the fact that during the manufacturing process of alloyed galvanized steel sheets, which are welded and then heat treated, the diffusion rate of Fe from the base steel to the plated layer becomes non-uniform, resulting in a columnar alloy structure. Let be a characteristic coarse profile.

Therefore, the surface roughness of the plating layer is determined depending on the averaged Rms rather than directly on the surface profile of the plated original plate.
After plating, by heat rolling or the like, dull grains are superimposed and transferred, and excessive eccentricity of the surface profile is corrected. It is thought that the roughness of the original plate has little direct effect on the continuous dotability of welding, and even when the plating layer melts and the electrode tip approaches the original plate surface, the electrode surface is still covered with the plating layer. It is recognized that there is.

By doing so, excellent effects such as improved welding workability of the alloyed galvanized steel sheet and dramatically expanding the uses of the galvanized steel sheet can be obtained.

Next, examples of the present invention will be listed together with comparative examples.

As described above, the present invention showed excellent weldability.

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram showing the relationship between continuous dots (weldability) and surface roughness of an alloyed galvanized steel sheet.

Claims (1)

[Claims] 1 The surface roughness of the alloyed galvanized steel sheet is 1.3 to 3.5.
Alloyed galvanized steel sheet with excellent weldability formed into μ-rms.